From: "Reece Dunn" <msclrhd@hotmail.com>
David Abrahams writes:
I think your definitions are mostly on target, but I think where you go wrong is that traits/policies has less to do with how a template is defined than how it's used.
To me a traits class provides information to a template class that gives (possibly platform or type dependant) information. For example, the decimal point character (100.00 or 100,00?) would be in a traits class (I know this is in a locale, but this is an example). Another example would be a format string to use as a separator between items in a list, depending on whether a character, string, wide character, wide string, etc are used.
A policy class provides functional, i.e. behavioural, information. Examples include string comparison, pointer validation (for smart pointers) and error handling (e.g. smart pointers and how to handle InvalidPointer - nothing, throw std::exception, assert failure, etc.).
I don't agree with those distinctions. I don't have a problem with a traits class providing behavior so long as that behavior, specialized as appropriate for the type, is the only behavior you can get using that traits name. That is, when you want character traits as defined by the Standard Library, you don't take a template parameter supplying the traits class, you simply use std::char_traits<T>::foo. When your template requires clients to provide overriding functionality, you add a policy parameter through which clients can inject the class that best implements the policy semantics for their purposes. To me, policies are passed to a template as a parameter, traits are simply known by their name within the template. (Gee, why didn't I just say *that* before!) -- Rob Stewart stewart@sig.com Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer;